The project is designed to provide a detailed description of the brain of the horseshoe crab with emphasis on the optic centers, their interconnections and synaptic architecture (synapse). Subjects to be investigated include the ganglia of the compound eye, i.e., lamina and medulla; ganglia of the median eye; the central body, an integration area for all visual inputs; and the corpora pedunculata, a massive lobe of the brain engaged in the processing of all sensory modalities. Specific problems relate to the distribution of neurosecretory cells (neurosecretion) in the protocerebrum and their relationship to efferent ocular innervation; synaptic contacts of eccentric and retinula cells iL the lamina; functional morphology of neuroglial cells (glia) in the central nervous syst@m; and the quantitation of fiber tracts to the corporapedunculata as it relates to synaptic devergence in the glomeruli and convergen@e in the neuropil of the peduncles. The study will utilize light microscopic methods of serial sectioning, reconstructions, and scanning electron microscopy (SEM) for general anatimical orientation, but primarily electron microscopy (EM) in the analysis of neural networkd and synaptic regions, butteressed by Golgi impregnations, selective staining techniques, and degeneration methods to enhance the specificity of identifications. functional interpretations will take cognizance of the wide spectrum of available physiological studies on thevisual system and should make this brain an attractive object for neurophysiological exploration.